1. Field of the Invention
This invention relates to spark plug equipped with a pressure sensor to detect pressure appearing in a combustion chamber at the time of running an engine so as to transmit it to a micro computer in a form of electrical signal.
2. Description of the Prior Art
This kind of spark plug has been introduced to detect pressure in a combustion chamber. This pressure is transformed into electrical signal by a pressure sensor which controls a timing of ignition and fuel injection with an assist of a micro computer for an optimum output and fuel expense.
The sensor has usually a piezoelectrical element enclosed into a dough-nut shaped casing made from metallic material. The casing is inserted into a diameter-reduced portion of a spark plug to be substantially in integral with the plug.
In this instance a sealant such as for example a fluororubber O-ring is provided between an outer side of the diameter-reduced portion and an inner side of the casing to maintain air-tight relationship therebetween.
At the time of mounting the spark plug, the plug is secured to a cylinder head with the casing tightly sandwiched between a stepped portion of the diameter-reduced portion.
This enables to transmit a precise pressure in a combustion chamber to the piezoelectrical element of the sensor.
The expensive O-ring however is exposed to adverse environment of extremely high temperature so that the O-ring is liable to deteriorate with a long usage of time.
Therefore it is a primary object of this invention to provide a spark plug with pressure sensor which is capable of maintaining a precise pressure detecting function for a long period of time with a relatively simple construction.
Accordingly there is provided a spark plug with pressure sensor comprising; a center electrode having a high voltage terminal; an insulator provided around said center electrode to securely support it; a tubular metallic shell provided to support said insulator at its inner side; a pressure sensor having a piezolectrical element at least enclosed by a casing of a metallic hollow ring; said metallic shell having a diameter-increased portion and a diameter-reduced portion; said casing of said sensor being inserted into said diameter-reduced portion with an inner side of said casing tightly engaging said outer side of said diameter-reduced portion; a mount means having a ring portion and a flange portion; said ring portion tightly inserted into said diameter-reduced portion while said flange portion tightly sandwiching said casing with a step between said diameter-reduced portion and said diameter-increased portion; and said ring portion being welded to said diameter-reduced portion to maintain air-tight relationship between an innerside of said ring portion and outer side of said diameter-reduced portion.